Industrial, chemical and mining processes resulting in acid deposition have had a significant negative effect on aquatic resources in North America, including the loss of important commercial and recreational fisheries. Direct effects of acidification on fish include acute mortality, reproductive failure, altered growth rates, and chronic impairment to body organs and tissues. Negative indirect effects of acidification include fish habitat degradation, increase in concentrations of soluble toxic metals, such as aluminum, and changes in predator-prey relationships. Within the coal deposit regions of Appalachia and the Ohio River Basin, acid mine drainage (AMD) contributes significantly to acid deposition in surface waters. See Table 1
TABLE 1 ______________________________________ Miles of streams degraded by AMD in the Appalachian coal region (Pennsylvania Department of Environmental Protection, 1995) State Miles ______________________________________ Pennsylvania 2594 West Virginia 1900 Maryland 156 Ohio 852 Kentucky 1129 Virginia 101 Tennessee 698 Alabama 626 ______________________________________
In Pennsylvania alone, AMD has degraded 2,600 miles of streams resulting in an annual loss of revenues associated with sport fishing of 67 million dollars. Given the severity of the problem and associated environmental/economic ramifications, the National Biological Service (NBS) in February 1995 signed the statement of Mutual Intent for "Restoration and Protection of Streams and Watersheds Polluted by Acid Mine Drainage from Abandoned Coal Mines," put forth by the Office of Surface Mining and the Environmental Protection Agency. Work has been done to increase the understanding and application of the best technologies available for remediating and preventing mine drainage and to support the development of new technologies.
Acid mine drainage (AMD) results from the dissolution of pyrite and its subsequent oxidation to sulfuric acid: EQU FeS.sub.2 +H.sub.2 O+3.5 O.sub.2 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4( 1)
Sulfuric acid dissolves aluminum, manganese, zinc, and copper from soil, and thus drainage is not only highly acidic but it may contain toxic metallic ions. Mitigation of AMD is typically achieved through direct addition of alkaline materials followed by clarification. High costs, however, limit widespread application of treatment. For example, with currently available technology, it has been estimated that 15 billion dollars will be required to correct AMD-related problems in Appalachia and 5 billion dollars in Pennsylvania. Alkaline materials used to treat acidified water include anhydrous ammonia, sodium hydroxide, sodium carbonate, calcium hydroxide, calcium oxide, and limestone. Limestone is a shaly or sandy sedimentary rock composed chiefly of calcium carbonate. Use of limestone (calcium carbonate) is desirable given its relatively low cost and widespread availability. Moreover, limestone is less caustic than alternative reagents; thus, use of limestone reduces the hazards of handling and application. Limestone dissolution also provides calcium ions needed to reduce the toxicity of certain dissolved metals. However, limestone use is restricted to sites with low acidities due to the slow dissolution (acid neutralizing) rates and problems associated with the development of a metal hydroxide coating of the limestone particles (armoring).
A variety of processes and apparatus designs using calcium carbonate have been developed in an attempt to treat effluent acidity.
In U.S. Pat. No. 4,272,498, Faatz discloses a non-mechanical method of converting coarsely ground limestone to a very fine powder. A slurry of this ground limestone is then contacted with carbon dioxide gas at high pressure to convert the solids in the slurry to an unstable form. The carbon dioxide pressure is instantaneously released to form a slurry of activated calcium carbonate particles substantially reduced in size. The activated calcium carbonate slurry is then used to scrub flue gases.
In U.S. Pat. No. 2,642,393, H. W. Gehm et al. discloses a neutralizing unit for a plant or system for the neutralization of acidic liquids. The arrangement effects an upflow of acid waste through a filter bed of a neutralizing agent of solid particles that are maintained in suspension and continually agitated. Air is used to affect a further suspension and agitation of the limestone particles, but does not chemically effect the reaction between the effluent and the limestone. There is no disclosure of carbon dioxide pretreatment of the effluent to increase limestone dissolution, pulsed bed technology which decrease the system's sensitivity to limestone armoring, or the recycling of CO.sub.2 gas.
In U.S. Pat. No. 1,742,110, C. R. Weihe discloses the use of a neutralizing agent to be maintained in contact with the running stream of waste water in such a way that the treating agent does not pass out with the waste water in the stream. The invention is particularly adapted for use in connection with neutralizing waste waters from mines, mills, and factories before it enters streams, lakes or rivers.
In U.S. Pat. No. 3,527,702, Holluta et al. disclosed a method of removing carbon dioxide from water using Portland cement clinkers or set hydraulic cements which consist of calcium oxide, silica and iron-aluminum oxides.
In U.S. Pat. No. 4,153,556, Riedinger discloses an apparatus for conditioning "aggressive" demineralized brackish water or sea water to remove CO.sub.2 and raise the pH to about 8. A wide angle, low pressure (approximately 10-20 psi) spray nozzle is used to supply purified aggressive water to the system so that it can percolate up through the limestone bed and pass out through an outlet pipe to a final conditioned water storage tank.
In U.S. Pat. No. 5,158,835, Burke discloses blocks weighing about 35 lbs., formed of a homogenous mixture of about 75% gypsum and 25% lime. The blocks are strategically placed in surface water that is being damaged by acid rain and where by timed release of lime, the pH of the water is maintained at about 6.5.
In U.S. Pat. No. 5,484,535, Downs discloses a method for treating effluent seawater including aerating the effluent seawater in an aeration pond. The aerated effluent seawater is then channeled back to the fresh seawater source. Fresh limestone is added periodically to the bed and the size of the bed is varied depending on the amount of effluent seawater to be treated.
In U.S. Pat. No. 5,487,835, Shane discloses a method and apparatus for controlling the pH of a water stream using carbon dioxide. Carbon dioxide at a selected pressure and flow rate is mixed with the carrier water also at a selected pressure and flow rate. The carbon dioxide-carrier water mixture is injected into the water stream, which is at a lower pressure, allowing the carbon dioxide to come out of the solution, contact the water stream and correspondingly adjust the pH of the water stream.
In addition, a variety of apparatus designs have been used to dose AMD with calcium carbonate. These include a rotary drum, electric powered dosers, packed beds, and a diversion well. The diversion well has been applied with relatively low initial capital and maintenance costs at several Pennsylvania AMD sites. The diversion well is designed to establish a fluidized bed of crushed limestone 6-25 mm in diameter. Fluidization occurs within a cylindrical well that receives water through a centrally located down pipe discharging water at the bottom of the well. The diverted water flows upward through the limestone with sufficient force to agitate and fluidize the medium causing abrasion of the aggregate for enhanced dissolution. Although the device provides low total costs of treatment, treatment effect is severely limited by the use of relatively large aggregate diameters and high required hydraulic loading rates. The large aggregate diameters are used to circumvent problems such as a slow dissolution rate, associated with metal hydroxide coating of the limestone. This coating or armoring of the medium occurs rapidly when treating waters with high ferrous iron (Fe.sup.++) concentrations.
Therefore, in spite of numerous attempts to restore water degraded by acid mine drainage and industrial chemical processes, there still remains a need for an improved process and apparatus using carbon dioxide pretreatment of the effluent to accelerate limestone dissolution with subsequent recycling of the CO.sub.2 gas stripped or recovered from water exiting the apparatus.